Sealed classes and methods in C Sharp (C#) - Time & Space Complexity

Time Complexity: Sealed classes and methods

O(n)

Understanding Time Complexity

When we use sealed classes or methods, we stop further changes through inheritance. This affects how the program runs when it calls these methods.

We want to see how sealing changes the number of steps the program takes as it runs.

Scenario Under Consideration

Analyze the time complexity of calling a sealed method versus a virtual method.

public class BaseClass
{
    public virtual void VirtualMethod() { /* some work */ }
    public sealed void SealedMethod() { /* some work */ }
}

public class DerivedClass : BaseClass
{
    public override void VirtualMethod() { /* some work */ }
    // Cannot override SealedMethod
}

// Usage
BaseClass obj = new DerivedClass();
obj.VirtualMethod();
obj.SealedMethod();
    

This code shows a base class with a virtual method and a sealed method, and a derived class overriding the virtual method but not the sealed one.

Identify Repeating Operations

Look at what happens when methods are called many times.

• Primary operation: Calling methods on objects.
• How many times: Each method call happens once per call, but many calls can happen in a loop or program.

How Execution Grows With Input

Calling a sealed method runs the exact code without extra checks. Calling a virtual method requires checking which version to run.

Input Size (n)	Approx. Operations
10	10 method calls, each with small overhead
100	100 method calls, overhead adds up for virtual calls
1000	1000 method calls, virtual calls have more overhead than sealed

Pattern observation: Sealed methods avoid extra checks, so their call cost stays simpler as calls grow.

Final Time Complexity

Time Complexity: O(n)

This means the total work grows directly with the number of method calls, but sealed methods have less overhead per call.

Common Mistake

[X] Wrong: "Sealing a method changes how many times it runs or makes it faster in a way that changes overall time complexity."

[OK] Correct: Sealing only removes the option to override and avoids some checks, but the number of calls still grows with input size the same way.

Interview Connect

Understanding how sealed methods affect execution helps you explain performance choices clearly. It shows you know how code structure impacts running time.

Self-Check

"What if the sealed method was changed to virtual and overridden many times? How would the time complexity or overhead change?"

Practice

1. What does the sealed keyword do when applied to a class in C#?

easy

A. Prevents the class from being inherited by other classes.

B. Allows the class to be inherited multiple times.

C. Makes the class abstract and uninstantiable.

D. Enables the class to override methods from its base class.

Solution

1. Step 1: Understand the sealed keyword on classes

   The sealed keyword on a class means no other class can inherit from it.

2. Step 2: Compare options with this meaning

   Only Prevents the class from being inherited by other classes. correctly states that the class cannot be inherited.

3. Final Answer:

   Prevents the class from being inherited by other classes. -> Option A

4. Quick Check:

   Sealed class = no inheritance [OK]

Hint: Sealed class means no child classes allowed [OK]

Common Mistakes:

• Thinking sealed classes can be inherited
• Confusing sealed with abstract
• Assuming sealed allows overriding

2. Which of the following is the correct syntax to declare a sealed method in C#?

easy

A. sealed public void MyMethod() { }

B. public override sealed void MyMethod() { }

C. public sealed void MyMethod() { }

D. override sealed public void MyMethod() { }

Solution

1. Step 1: Recall sealed method syntax

   A method can only be sealed if it overrides a base method, so it must have override sealed modifiers.

2. Step 2: Check options for correct order and modifiers

   public override sealed void MyMethod() { } correctly uses public override sealed void MyMethod(). Other options miss override or have wrong order.

3. Final Answer:

   public override sealed void MyMethod() { } -> Option B

4. Quick Check:

   Sealed method = override + sealed [OK]

Hint: Sealed methods must override and use 'override sealed' [OK]

Common Mistakes:

• Declaring sealed method without override
• Wrong order of modifiers
• Missing override keyword

3. What will be the output of the following code?

class Base {
    public virtual void Show() { Console.WriteLine("Base"); }
}
class Derived : Base {
    public sealed override void Show() { Console.WriteLine("Derived"); }
}
class MoreDerived : Derived {
    public override void Show() { Console.WriteLine("MoreDerived"); }
}

var obj = new MoreDerived();
obj.Show();

medium

A. Base

B. Derived

C. MoreDerived

D. Compilation error

Solution

1. Step 1: Understand sealed override effect

   The method Show in Derived is sealed, so it cannot be overridden in MoreDerived.

2. Step 2: Check the code for override in MoreDerived

   MoreDerived tries to override Show, which causes a compilation error.

3. Final Answer:

   Compilation error -> Option D

4. Quick Check:

   Sealed method cannot be overridden [OK]

Hint: Sealed override blocks further overrides causing errors [OK]

Common Mistakes:

• Assuming MoreDerived.Show runs
• Ignoring sealed keyword effect
• Thinking output is Derived

4. Identify the error in this code snippet:

sealed class Animal {
    public void Speak() { Console.WriteLine("Animal speaks"); }
}
class Dog : Animal {
    public void Speak() { Console.WriteLine("Dog barks"); }
}

medium

A. Method Speak must be virtual in Animal.

B. Cannot declare method Speak in Dog class.

C. Cannot inherit from sealed class Animal.

D. No error, code runs fine.

Solution

1. Step 1: Check sealed class inheritance rules

   A sealed class cannot be inherited by any other class.

2. Step 2: Analyze Dog class inheritance

   Dog tries to inherit from sealed Animal, which causes a compilation error.

3. Final Answer:

   Cannot inherit from sealed class Animal. -> Option C

4. Quick Check:

   Sealed class blocks inheritance [OK]

Hint: Sealed class cannot have child classes [OK]

Common Mistakes:

• Thinking method override causes error
• Ignoring sealed class inheritance rule
• Assuming code compiles fine

5. You have a base class Vehicle with a virtual method Start(). You want to create a class Car that overrides Start() but prevents any further subclass from overriding it. How should you declare Start() in Car?

hard

A. public override sealed void Start() { }

B. public sealed void Start() { }

C. public override void Start() sealed { }

D. sealed public override void Start() { }

Solution

1. Step 1: Understand method sealing rules

   To prevent further overrides, the method must be both override and sealed.

2. Step 2: Check correct syntax for sealed override

   The correct syntax is public override sealed void Start(). Other options have wrong order or missing keywords.

3. Final Answer:

   public override sealed void Start() { } -> Option A

4. Quick Check:

   Sealed override method syntax = override sealed [OK]

Hint: Use 'override sealed' to block further overrides [OK]

Common Mistakes:

• Omitting override keyword
• Wrong order of sealed and override
• Trying to seal without overriding